The invention relates to intervertebral disk prostheses.
European Patent No. 277 282-Al discloses an intervertebral disk prosthesis, comprising two plates and a cushion interposed between them. The cushion comprises a compressible body delimiting a cavity filled with an incompressible fluid. This prosthesis is essentially incompressible in the axial direction and allows only a relative inclination of the plates. This behavior is different from that of a healthy, natural intervertebral disk.
An object of the invention is to provide a disk prosthesis which more closely imitates and approximates the mechanical properties of a healthy, natural intervertebral disk.
With a view to achieving this object, according to the invention an intervertebral disk prosthesis is envisaged comprising two plates and a cushion interposed between the plates, the cushion comprising a compressible body and containing a fluid, which is compressible.
Thus the compression of the cushion affects the compression of the body and the fluid. Since the compression properties of the body and the fluid can be different, their combination allows a very close approximation to the mechanical properties of a healthy, natural intervertebral disk. More particularly, when the body is made of suitable material, the curve of the mechanical reaction to a compression of the cushion as a function of a variation in a dimension of the cushion in the direction of compression can be obtained, having a hysteresis shape close to that associated with a healthy, natural disk.
Advantageously, the fluid has a pressure such that it is more compressible than the body.
This difference can thus be utilized to approximate as closely as possible the mechanical properties of the healthy, natural disk.
Advantageously, the fluid comprises a gas.
Advantageously, the cushion is arranged such that a fluid pressure is applied directly to the plates.
Advantageously, the fluid extends around the periphery of the body.
Advantageously, the body comprises a viscoelastic material, preferably silicone.
The aforementioned curve, thus has a highly pronounced hysteresis shape can thus be obtained.
Advantageously, the body is in contact with the plates.
Advantageously, the body has at least one end having a contact zone with one of the plates, the prosthesis being arranged such that the contact zone has a surface area which increases whenever a stressing of the plate in the direction of the body is increased.
For the lowest compression values, the mechanical reaction of the prosthesis upon the compression of the body thus varies very little as a function of the dimensional change in the cushion in the direction of compression. In other words, the aforementioned curve is little inclined relative to the horizontal for low compression values and little force is provided in the initial operation. This property reproduces that of a healthy, natural disk.
Advantageously, the contact zone is defined by a face of the plate and an end face of the body, one of the two faces, typically the face of the body, being curved and convex and the other face being flat.
Advantageously, the contact zone is defined by a face of the plate and an end face of the body, the two faces being curved in at least one common direction and being respectively concave and convex, the concave face having at least one radius of curvature greater than a corresponding radius of curvature of the convex face.
As a result of this configuration, the variations in mechanical reaction, previously mentioned, can be effected. Moreover, when the body is free to shift laterally relative to the plate, as will be seen later, this configuration guarantees the relative centering of the two faces. For example, after the two faces have been mutually offset, these curvatures enable them to re-center automatically.
Advantageously, the body has at least one end in contact with one of the plates, this end being free to move relative to the plate in a direction parallel to the plate.
Advantageously, the end is accommodated in a recess of the plate and forms a lateral abutment for the body.
The lateral displacements of the body relative to the plates can thus be limited, or even barred.
Advantageously, the cushion comprises a shell containing the fluid and arranged such that it has a cross-sectional area parallel to the plates which is essentially invariable when variation occurs in a compression of the cushion between the plates.
Advantageously, the cushion comprises a chamber containing the fluid and extending around the periphery of the body at a distance.
The erosion of the body by the chamber in the course of its movement and the dispersion of particles of the body are thus prevented.
Advantageously, the chamber forms a spring, especially a compression spring.
The chamber thus influences the reaction of the cushion whenever this is compressed.
Advantageously, the cushion is arranged to exhibit a hysteresis-shaped curve of mechanical reaction to a compression as a function of a variation in a dimension of the cushion in the direction of the compression.
Advantageously, the cushion is arranged such that the reaction to the compression grows less markedly for relatively low reaction values than for relatively high reaction values.
Advantageously, the cushion is arranged such that the reaction to the compression diminishes more markedly for relatively high reaction values than for relatively low reaction values.
Advantageously, the cushion is arranged such that the reaction to the compression has higher values when it grows than when it diminishes.
Advantageously, the prosthesis is intended for the lumbar region of the spine.
Other characteristics and advantages of the invention are yet to appear in the following description of the preferred embodiments, given by way of non-limiting examples.